The present invention relates to a data recording/reproducing apparatus using an optical disk or the like and, more particularly, to a tracking control apparatus therefor.
Data recording/reproducing apparatuses using optical disks have been commercially available in recent years. In order to read out data from an optical disk, a data recording track (to be referred to as a track hereinafter) is irradiated with a laser beam, and data is reproduced based on the beam reflected by the track. When the tracks are helically formed on the optical disk, since the sectors of a single track are not equidistant from the center of rotation of the disk, tracking control is necessary in the read mode to accurately irradiate the track with a laser beam. Even if the tracks are concentrically formed, sectors of a single track are not equidistant from the center of rotation of the disk due to eccentricity in the disk, thus making tracking control necessary.
A typical conventional example of a tracking circuit is a lens tracking circuit which moves an objective lens in response to a tracking error signal derived from light reflected by or transmitted through an optical disk. The objective lens is normally supported and fixed by a spring on an optical head housing. An actuator is energized to move the lens. When the actuator is deenergized, the lens is held at a mechanically neutral point balanced by a spring force. However, when track eccentricity exceeds twenty or thirty microns, the objective lens deviates greatly from the mechanically neutral point, thereby mixing an optical offset signal with the tracking error signal. The laser beam then traces an incorrect track in response to the optical offset signal.
In order to eliminate such an optical offset signal, a tracking system called a two-step servo system has been developed, as described in Japanese Patent Disclosure No. 60-143442. According to this system, the carriage and hence the optical pickup itself as well as the objective lens are moved to perform cooperative tracking. A tracking error signal representing a tracking error is supplied to the objective lens actuator and a carriage moving voice coil motor.
Gain G.sub.V of the carriage tracking circuit for the tracking error signal is larger than gain G.sub.L of the lens tracking circuit therefor. When carriage tracking is started with a large tracking error signal, the voice coil motor is excessively operated. Tracking with a laser beam becomes more difficult. For this reason, lens tracking comes first in the conventional two-step servo system. After the tracking error signal is reduced, carriage tracking is started.
The two-step servo tracking system, however, has the following drawback.
The frequency response of the carriage tracking circuit including the voice coil motor is shown by the broken line in FIG. 1. Its gain reaches a maximum in response to a low-frequency tracking error signal, i.e., low-frequency variations, and the gain is reduced when the frequency is increased. However, since the objective lens is supported and fixed by a leaf spring or the like, the lens is affected by the eigenfrequency of the leaf spring. The frequency response of the lens tracking circuit is shown by a solid line in FIG. 1.
Therefore, when objective lens tracking is started first as in conventional two-step servo tracking, the response at the low-frequency variations is poor. As a result, it is difficult to provide satisfactory tracking control.